1. Field of the Invention
The present invention relates to a tilt servo controlling apparatus for correcting a tilt error of an optical disc.
2. Description of the Related Art
In the technical field of an optical disc such as a CD (Compact Disc), a DVD or the like, there is a problem that an angle between an optical axis of a light beam emitted by an optical pickup and a surface of the optical disc at an irradiation position (i.e., a tilt angle) is shifted from a perpendicular angle, due to a distortion or a surface deflection of the optical disc. The tilt angle is generated mainly in a radial direction of the optical disc and becomes a factor of a comma aberration of the optical system, which gives a harmful influence onto a reproduction quality of the optical disc since it causes a cross talk between adjacent tracks and a deterioration of a jitter. Further, if a high density recording is attempted such as a DVD, since the spot diameter of the laser beam becomes small, it is necessary to shorten the wavelength .lambda. of the laser beam and enlarge a numerical aperture of an objective lens, so that a margin with respect to the tilt angle becomes small. Namely, even if the optical disc is tilted in a very small degree, a large deterioration in the reproduction quality is generated. Therefore, in order to correct the aberration due to the tilt angle during the reproducing operation of the optical disc, there are installed (i) a tilt sensor including a light source of an exclusive light beam to detect the tilt angle and a detector to detect the exclusive light beam, and (ii) a mechanism for controlling the inclination of the optical pickup in correspondence with the detected tilt angle, so as to apply the tilt servo control.
However, according to the above explained tilt servo control, the tilt sensor, which has a detector and a light source to emit a pair of light beams, is necessary besides the optical pickup, and a complicated mechanical portion to control the tilt of the optical pickup is necessary. This leads to the cost up and makes it difficult to miniaturize the apparatus since it occupies a certain space.